Some embodiments described herein relate to electromagnetic machines and more particularly to devices and methods for increasing energy and/or power density in composite flywheel energy storage systems.
Electromechanical flywheel devices can be used for large capacity energy storage to improve, for example, the economic performance and stability of utility, industrial, military, and/or other suitable grid infrastructures. Such flywheel devices are mechanical—storing energy via rotational kinetic energy and delivering energy back to the grid or local energized component via a motor/generator system at least electrically connected to the flywheel device. The application of some known flywheel energy storage systems, however, can be limited based at least in part on physical limitations associated with a mechanical system (e.g., high forces associated with rotational velocities and acceleration, which can lead to failure of component materials and/or catastrophic system failure and/or the like).
For example, it is usually desirable to maximize the energy density (energy per unit mass, W-h/kg). The kinetic energy associated with the flywheel can be increased (e.g., added or inserted) by application of electrical energy, or decreased by extraction of electrical energy, via a motor-generator that is operably coupled to and/or otherwise included in the primary energy storage portion of the device. One way to increase energy per unit mass of a flywheel is to form the flywheel, at least in part, from high-strength, low density composite material (e.g., carbon fiber. Because carbon fiber has a higher tensile strength per unit mass than other materials (such as glass fiber or steel), a flywheel formed from carbon fiber can rotate at a relatively higher rotational velocity (due to higher tensile strength to resist circumferential stresses) for a given amount of mass, thus increasing the rotational kinetic energy for that amount of mass, i.e. density per unit mass. However, composite materials, such as those formed from carbon fiber, have much lower strength in the radial direction than in the circumferential direction because radial stresses are carried by the composite's matrix material, e.g. a polymer resin. The matrix material has much lower tensile strength than the fiber material (e.g. carbon fiber). Thus, the rotational velocities of flywheels formed of carbon fiber are limited by the strength of the matrix, rather than the strength of the carbon fiber.
Thus, a need exists for devices and methods for changing the relationship between radial and circumferential stresses in flywheels formed of high-strength composite materials to enable increased energy and/or power density of the flywheel.